In-mold foam molded articles obtained using polypropylene resin foamed particles have characteristics, which are advantages of the in-mold foam molded article, of shape arbitrariness, lightness, thermal insulation and so on. In addition, the in-mold foam molded article obtained using the polypropylene resin foamed particles is superior in chemical resistance, heat resistance, and strain recovery ratio after compression to in-mold foam molded articles obtained using polystyrene resin foamed particles, and superior in dimensional accuracy, heat resistance, and compressive strength to in-mold foam molded articles obtained using polyethylene resin foamed particles.
Owing to the characteristics described above, the in-mold foam molded article obtained using the polypropylene resin foamed particles is used in various uses such as automobile interior members, cores for an automobile bumper, insulating materials, shock absorbing packaging materials, and returnable boxes.
For example, Patent Document 1 discloses polypropylene resin pre-foamed particles using, as a base material resin, a polypropylene resin having a specific melt-flow rate, a specific melting point, and a specific flexural modulus and also describes a returnable box obtained from the polypropylene resin pre-foamed particles.
In particular, in the returnable box, when heavy parts such as car parts are conveyed, it is desirable that the rigidity is high, the weight of the box itself is as light as possible, and as many as parts can be conveyed.
Then, a method for producing a returnable box is adopted which uses polypropylene resin foamed particles whose base material resin is a high rigid polypropylene resin having a bending elastic modulus of 1300 MPa or more. During in-mold foam molding of the polypropylene resin foamed particles, a heated steam pressure tends to be increased; as a result, a molding cycle in which the polypropylene resin foamed particles are filled in a mold, the particles are fused with heated steam, the resulting product is cooled, and the obtained in-mold foam molded article is taken out from the mold tends to be long.
It is often to provide a grip structure on the returnable box so that workers easily carry it, and, a returnable box for conveying heavy parts has preferably a grip structure which can be securely grasped by inserting workers' fingers, the structure being not a structure to merely hang workers' fingers (for example, Patent Document 2).
When the grip structure part is obtained by the in-mold foam molding, however, depressions and the like looking like pockmarks are generated around a grip structure part on a molded article surface and surface properties are problematically reduced, because it is difficult to pass heated stream or to cool effectively due to a mold structure.
Polypropylene resin pre-foamed particles having a ratio of a Z-average molecular weight Mz to a number average molecular weight Mn, Mz/Mn, which is an indicator of a molecular weight distribution, of 16 or less are disclosed (for example, Patent Documents 3 and 4). These Patent Documents describe that when polypropylene resin pre-foamed particles having an Mz/Mn ratio of 16 or less are used, the obtained platy molded article, or the obtained molded article having protrusions formed on its surface, have the excellent surface nature, but they do not describe or suggest the molding cycle, or the surface properties of the part around the grip of the returnable box having the grip structure.
On the other hand, methods for producing a polypropylene resin having an Mz/Mn ratio of 20 or more are disclosed (for example, Patent Documents 5 to 9), and the production technique of the polypropylene resin having an Mz/Mn ratio of 20 or more is a known technique. However, even if the polypropylene resin foamed particles are produced merely using the polypropylene resin having an Mz/Mn of 20 or more, a returnable box molded article having a short molding cycle and excellent surface properties cannot be obtained.
Although Patent Document 10 does not disclose the Mz/Mn ratio, discloses a method for producing a polypropylene resin having a polydispersion index, which can be considered to be an indicator of a molecular weight dispersion, of 15 or more, propylene beads obtained from the polypropylene resin, and a foamed package for food which is obtained from the propylene beads. A numerical value of Mz/Mn, obtained from the average molecular weight, however, does not coincide with a numerical value of the polydispersion index, obtained from a storage elastic modulus or loss elastic modulus at melting, and there is no knowledge in which they are theoretically interrelated with each other. In addition, Patent Document 10 does not absolutely describe a molding cycle at the time when the propylene beads are subjected to the in-mold foam molding or the surface properties of an in-mold foam molded article. It is not always right that if any polypropylene resin is used, so long as the polypropylene resin has a polydispersion index of 15 or more, then the molded returnable box article can have the short molding cycle and the excellent surface properties. Patent Document 10 also describes a melt strength of 1.50 cN or more, but if the melt strength is too high, a polypropylene resin foamed particles having a high expansion ratio cannot be obtained, and the surface properties are remarkably reduced in the in-mold foam molding.